This invention relates to the olefination and subsequent molecular averaging of the waxy fraction resulting from Fischer-Tropsch synthesis.
The majority of distillate fuel used in the world today is derived from crude oil. Crude oil is in limited supply, includes aromatic compounds believed to cause cancer, and contains sulfur and nitrogen-containing compounds that can adversely affect the environment. For these reasons, alternative methods for generating distillate fuel have been developed.
One alternative method for generating distillate fuel involves converting natural gas, which is mostly methane, to synthesis gas (syngas), which is a mixture of carbon monoxide and hydrogen. The syngas is converted to a range of hydrocarbon products, collectively referred to as syncrude, via Fischer-Tropsch synthesis.
It is generally possible to isolate various fractions from a Fischer-Tropsch reaction, for example, by distillation. The fractions include a gasoline fraction (B.P. about 68-450xc2x0 F./20-232xc2x0 C.), a middle distillate fraction (B.P. about 250-750xc2x0 F./121-399xc2x0 C.), a wax fraction (B.P. about 650-1200xc2x0 F./343-649xc2x0 C.) primarily containing C20 to C50 normal paraffins with a small amount of branched paraffins and a heavy fraction (B.P. above about 1200xc2x0 F./649xc2x0 C.) and tail gases. 
An advantage of using fuels prepared from syngas is that they do not contain significant amounts of nitrogen or sulfur and generally do not contain aromatic compounds. Accordingly, they have minimal health and environmental impact.
However, a limitation associated with Fisher-Tropsch chemistry is that it tends to produce a broad spectrum of products, ranging from methane to wax. While the product stream includes a fraction useful as distillate fuel, it is not the major product.
Fischer-Tropsch products tend to have appreciable amounts of olefins in the light fractions (i.e., the naphtha and distillate fuel fractions), but less so in the heavy fractions. Depending on the specifics of the Fischer-Tropsch process, the naphtha can be expected to include more than 50% olefins, most of which are alpha olefins. Distillate fuels will also contain some level of olefins (typically between 10 and 30%) and the distillate waxy fractions can contain smaller quantities.
One approach for preparing distillate fuels is to perform Fischer-Tropsch synthesis at high alpha values that minimize the yield of light gases, and maximize the yield of heavier products such as waxes. The wax from the Fischer-Tropsch process typically causes the entire syncrude to be a solid even at high temperatures, which is not preferred. The waxes are then hydrotreated and hydrocracked to form distillate fuels. Since hydrocracking is performed at relatively high temperatures and pressures, it is relatively expensive.
It would be advantageous to provide a process which provides useful distillate fuels from Fischer-Tropsch products but which does not require a hydrocracking step. The present invention provides such a process.
In its broadest aspect, the present invention is directed to an integrated process for producing distillate fuels, including jet fuel, gasoline and diesel. The process involves the partial dehydrogenation of the wax fraction and/or heavy fraction of a Fischer-Tropsch reaction to form olefins, which are reacted with the olefins in the naphtha and/or light gas fraction of the Fischer-Tropsch reaction in the presence of an olefin metathesis catalyst. The resulting product has significantly less wax, and the product has an average molecular weight between the molecular weight of the naphtha and/or light gas fractions and the molecular weight of the wax and/or heavy fractions.
Fractions in the distillate fuel range can be isolated from the reaction mixture, for example, via fractional distillation. The product of the molecular averaging reaction tends to be highly linear, and is preferably subjected to catalytic isomerization to improve the octane values and lower the pour, cloud and freeze points. The resulting composition has relatively low sulfur values, and relatively high octane values, and can be used in fuel compositions.
In one embodiment, one or both of the feeds to the molecular averaging reaction is isomerized before the molecular averaging reaction. Incorporation of isoparaffins into the molecular averaging reaction provides a product stream that includes isoparaffins in the distillate fuel range which have relatively high octane values.
In another embodiment, the alpha olefins in the light naphtha and gas are converted into internal olefins (either normal internal or iso-internal olefins). When these materials are averaged against the internal olefins derived from dehydrogenation of the wax, the yield of intermediate fuels is increased. Furthermore, the light naphtha and gas fractions may contain impurities such as alcohols and acids. These oxygenates can be converted to additional olefins by dehydration and decarboxylation. Traces of other impurities should be reduced to acceptable levels by use of adsorbents and/or extractants.
Preferably, after performing Fischer-Tropsch synthesis on syngas, and before performing the molecular averaging reaction, hydrocarbons in the distillate fuel range are separately isolated, for example, via fractional distillation. The wax and/or heavy fraction are then dehydrogenated, the naphtha and/or light gas fractions are added to the resulting olefinic mixture, and reaction mixture is molecularly averaged by subjecting the olefins to olefin metathesis conditions.
It is preferred that the wax and/or heavy fraction and the naphtha and/or light gas fractions are derived from Fischer-Tropsch synthesis. However, at least a portion of the low molecular weight olefins or the waxy fraction can be derived from a source other than Fischer-Tropsch synthesis. Due to the nature of the molecular averaging chemistry, the reactants cannot include appreciable amounts (i.e., amounts that would adversely affect the catalyst used for molecular averaging) of thiols, amines, or cycloparaffins.
It may be advantageous to take representative samples of each fraction and subject them to molecular averaging reactions, adjusting the relative proportions of the fractions until a product with desired properties is obtained. Then, the reaction can be scaled up using the relative ratios of each of the fractions that resulted in the desired product. Using this method, one can xe2x80x9cdial inxe2x80x9d a molecular weight distribution which can be roughly standardized between batches and result in a reasonably consistent product.